This invention relates to a device utilizing a laser beam for target detection, and more particularly to such device employing heterodyne detection of the target echoes.
In laser radar devices heterodyne detection has been found to be useful in increasing the signal to noise ratio of the system in view of the gain involved in the mixing process. A typical prior art implementation of a laser system utilizing heterodyne detection is shown in FIG. 1. In this implementation, a beam from laser 11 is passed through beam splitter 12 to targets, echoes from the targets being reflected from a surface of beam splitter 14 onto detector 15. A portion of the laser energy is also reflected from the surface of beam splitter 12 and passes through beam splitter 14 to the target. The two beam splitters are aligned such that the wave fronts of the beam from the targets, reflected off the top surface of beam splitter 14, and the beam from the laser which passes through beam splitter 14 are parallel. It can be shown under such conditions, the electrical signal, S.sub.h, obtained from the detector is as follows: EQU S.sub.h = R.sqroot.P.sub.LO P.sub.s ( 1)
where
R = detector responsivity PA1 P.sub.lo = local oscillator power PA1 P.sub.s = signal power.
Thus it can be seen that a substantial improvement in power output is achieved over the detection process referred to as incoherent detection where no mixing of the target signal is utilized, in which case the detector power is equal to RP.sub.s. Heterodyne detection systems of the prior art such as that of FIG. 1, however, require precise alignment of the optical components for proper operation for, as already noted, the two beam splitters of FIG. 1 must be aligned such that the wave fronts of the two beams striking the detector are parallel to within a small fraction of the diffraction angle of the receiver or transmitter optics. Such alignment is difficult to maintain, particularly where the system is carried on a vehicle subject to jarring, shock and vibration, as is generally the case in military applications. Further, the use of beam splitters presents losses of light energy which undesirably lowers the efficiency of the system.
The present invention overcomes the aforementioned shortcomings of the prior art in providing a system which is relatively insensitive to alignment errors such as might be occasioned by jarring or vibration of system components. Further, the system of the invention utilizes reflective elements throughout so as to avoid the inherent inefficiencies of partially transmissive elements such as beam splitters.
It is therefore an object of this invention to lessen the requirement for precise alignment of elements in a radar laser heterodyne detection system.
It is a further object of this invention to facilitate the operation of a laser radar system using heterodyne detection in a vehicle which is subject to vibration and shock.
It is still another object of this invention to increase the efficiency of operation of a laser radar using heterodyne detection.
Other objects of the invention will become apparent as the description proceeds in connection with the accompanying drawings.